1. Field of the Invention
The present invention relates to the field of fluid flow measurement, and in particular to measurement of fluid flow by means of thermal mass flow meters.
2. Prior Art
As shown in FIG. 1, TMFMs have two sensors immersed in a flowing fluid--a thermal mass flow sensor (TMFS) that monitors the mass flow of the flowing fluid and a temperature sensor that monitors the fluid's temperature and automatically corrects for temperature changes. In most current applications of TMFMs the fluid is a gas. Therefore, hereinafter the term "fluid" shall be referred to as "gas". In most conventional TMFMs, each sensor is a reference-grade platinum resistance temperature detector (RTD) wound on a ceramic mandrel and inserted into a tubular corrosion-resistant metallic sheath, or thermowell. The TMFS's RTD has a much lower resistance than the temperature sensor's RTD and is self-heated by the electronics to a temperature above the gas temperature, thereby creating the potential for the transfer of heat from the TMFS to the flowing gas stream. As will be shown later, the quantity directly monitored by TMFMs is the mass velocity (.rho.V), where .rho. is the mass density of the gas and V is its velocity. Conventionally, the electronics is an analog bridge circuit, but it can also be an all-digital, microprocessor-based sensor drive, as shown in FIG. 2.
FIGS. 3, 4, and 5 show typical configurations of TMFMs. The single-point insertion TMFM shown in FIG. 3 is inserted into ducts, stacks, and pipes. The mass flow rate (m) in the channel is measured as the product of the monitored quantity (.rho.V) and the cross-sectional area (A) of the pipe or duct, that is, m=.rho.VA
As shown in FIG. 4, multi-point insertion TMFMs in arrays of typically 2 to 50 sensors monitor the total mass flow rate (m) in large ducts, stacks, and pipes. Multi-point TMFMs are configured by dividing the channel's cross-sectional area (A) into n equal areas and locating a thermal sensor pair at the centroid of each equal area (A/n). Thus, the total mass flow (m) in the channel is computed by the electronics as: ##EQU1## where (m.sub.1) and (.rho.V), are the mass flow rate and mass velocity monitored by each sensor, respectively.
In-line TMFMs, like the flanged unit shown in FIG. 5, monitor the mass flow of gases in pipes or tubes. In-line flow meters typically are calibrated directly in terms of mass flow rate (m) with the actual gas.